1. Field of the Invention
The present invention relates to a single-fiber bidirectional optical module including a transmitter, a receiver, and a receptacle for bidirectionally transmitting a signal through a single optical fiber. In the bidirectional optical module, the present invention particularly has objects to suppress forward radiated noise, to reduce crosstalk between a laser diode (LD) and a photodiode (PD), and to reduce outer noise.
2. Description of the Background Art
In optical communication for bidirectionally transmitting an optical signal by using a single optical fiber, an optical transmission and reception module is provided at opposite ends of the optical fiber. According to a conventional optical transmission and reception module, an optical fiber is drawn from an LD module which stores an LD in a metal package, and another optical fiber is drawn from a PD module which stores a PD in a metal package. Then, the two optical fibers are joined together by a wavelength multiplexer demultiplexer to be connected to another optical fiber. Therefore, the conventional optical transmission and reception module includes three components, i.e., the LD module, the PD module, and the wavelength multiplexer demultiplexer, which are joined to one another by the optical fibers. The module is the combination of the separate components, and has the advantages of not suffering from the LD/PD crosstalk and being resistant to the outer noise. This configuration, however, increases the number of components and makes cost reduction difficult. As a result, a bidirectional module integrating all of the components is desired.
United States Patent Application Publication No. 2005-0053338 A1 proposes an integrated optical transmission and reception module in which a receptacle, a filter, an LD, and a PD are integrated. The module is a single-fiber bidirectional module and is also an integrated module, in which the LD and the PD are positioned perpendicularly to each other and the light is distributed through the filter. In this regard, the above module is in common with the module according to the present invention, and is therefore cited herein. In the module disclosed in the above publication, a package for storing the LD, a package for storing the PD, and a rectangular box for storing the entirety of the receptacle are in contact with one another, and are all at the same potential.
A metal container storing the above components is supplied with the anode potential or the cathode potential of the LD. As a powerful driving current of the LD generates electromagnetic waves, intense radio waves are generated with the container acting as an antenna. Thereby, strong electromagnetic noise is generated in a device and equipment located in front of the container. Further, a part of the powerful driving current of the LD flows into the PD and generates noise in the PD. Therefore, electrical and electromagnetic crosstalk between the LD and the PD is increased. Furthermore, according to the above configuration, noise is frequently generated in peripheral equipment.
The optical transmission and reception module integrating the LD, the PD, and the receptacle, such as the module proposed in the above publication, involves the forward radiated noise, the LD/PD crosstalk, the outer noise, and so forth. The first object of the present invention is therefore to provide an optical module having reduced forward radiated noise caused by the driving current of the LD. Further, the second object of the present invention is to provide a bidirectional optical module having reduced LD/PD crosstalk caused by the driving current of the LD. Furthermore, the third object of the present invention is to provide a bidirectional optical module having a receiver unaffected by the outer noise.
In the optical module according to the present invention, an insulative pipe connects a metal chassis including a transmitter and a receiver to a receptacle for removing an optical fiber. Thereby, the receptacle and the chassis are insulated from each other. Further, the potential of the metal chassis including the transmitter and the receiver is set at the potential of a receiver ground RG, while the potential of the receptacle is set at the potential of a frame ground FG. That is, the two separate grounds RG and FG are provided, and the chassis including the transmitter and the receiver is connected to the receiver ground RG, while the receptacle is connected to the frame ground FG.
The transmitter includes an LD for generating transmission light. The anode and the cathode of the LD are insulated from the metal chassis. To be at the receiver ground RG is to have the same potential as the potential of either one of the anode terminal and the cathode terminal of a light receiving element, i.e., a PD or an avalanche photodiode (APD). Since the light receiving element is reverse-biased to be used, the cathode thereof can be connected to the ground, for example. Conversely, the anode can be connected to the ground. The potential shared by the mutually connected chassis and either one of the cathode terminal and the anode terminal of the receiver is referred herein as the receiver ground RG. The anode and the cathode of the LD in the transmitter are not connected to the chassis, and are held at a floating potential. If the transmitter includes a monitor photodiode (MPD), the anode and the cathode of the MPD are also held at the floating potential.
In the transmission and reception module thus including the transmitter and the receiver, a single optical fiber propagates the transmission light and the reception light in opposite directions. Therefore, the module is referred to as the “single-fiber” module. In this case, the transmission light and the reception light need to be distinguished from each other by the wavelength. Therefore, a wavelength selective filter for selecting between the transmission light and the reception light in accordance with the wavelength is obliquely provided on an extension of the optical fiber at an angle of approximately 45°. The wavelength selective filter is also provided inside the metal chassis.
The present invention relates to a receptacle-type optical module. A receptacle is a jig capable of removing an optical fiber. An optical module fixed to the leading end of an optical fiber is referred to as a pigtail-type optical module. The optical module herein discussed is the receptacle-type optical module. The optical fiber used in this case is the single optical fiber inserted in the receptacle. Thus, the present optical module is referred to as the single-fiber bidirectional optical module.
The metal chassis in this case is practically a concept including a plurality of metal containers, and refers to metal members, such as a package for storing a light emitting element, a package for storing a light receiving element, and a holder for holding a wavelength selective filter. The metals are in contact with one another, and thus are at the same potential. In the conventional module, the receptacle and the metal chassis are connected to the same ground. According to the present invention, however, the chassis and the receptacle are electrically cut off from each other by the insulative pipe. The metal chassis and the receptacle are connected to the receiver ground RG and the frame ground FG, respectively. Both grounds are at the same potential in terms of direct current and are connected to each other in the distance. However, the path connecting the two grounds is long, and thus the two grounds have different potentials due to a resistance R and an inductive component L which effectively exist.
The driving current of the LD is large and rapidly changes. In some cases, therefore, the large and rapidly changing driving current of the LD causes an adverse effect on the light receiving element and an electrical element included in a frame, directly or in the form of radio waves. If the anode or the cathode of the LD is connected to the ground of the chassis, the chassis acts as an antenna. As a result, intense radio waves of the same frequency and shape as those of an LD driving signal propagate to the surroundings to cause the crosstalk between the light receiving element and to act on the electrical element in the frame to malfunction.